Make CONFIG_VIRTIO_BLK, CONFIG_VIRTIO_CONSOLE, CONFIG_BTRFS_FS and CONFIG_EXT4_FS builtin modules

Making these builtin modules allows booting disk images in virtual machines without needing an initramfs. We'd like to make use of this in systemd's integration test infrastructure to speed things up by not requiring an initrd for most integration tests. CONFIG_VIRTIO_CONSOLE is not necessary to make this work, but you don't get console output if things don't work without it so to simplify debugging I figure it makes sense to make that one builtin as well.